Cathode gun device

ABSTRACT

A cathode gun device having a heat shield for improved operating efficiency is disclosed. The device includes a tubular cathode electrode and a thin metal foil made of low thermal conductivity and low emissivity material coiled about the cathode electrode for reflecting heat back to the cathode and conducting a minimum of heat. The foil has projections on a surface so as to separate adjacent coil surfaces from each other.

Basiulis CATHODE GUN DEVICE [75] lnventor: Algerd Basiulis, Redondo Beach,.

Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: Apr. 9, 1973 [21] Appl. No.: 349,156

[52] US. Cl 313/326, 313/240, 313/242, 313/346, 3l5/3.5 [51] int. Cl H0lj 1/00, HOlk 1/02 [58] Field of Search 313/326, 240, 242, 239, 3l3/340, 346, 356

[5 6] References Cited v UNITED STATES PATENTS.

2,39l,927 1/1946 Segerstrom ..lr. 313/340 X 2,5l8,879 8/l950 Germeshausen....- 3l3/240 X 2,577,239 12/1951 Eitel et al. 313/340 4lo 4Ib 45'- 7 4th 47 2 46- r 44d e a 5] June 4, 1974 2,888,592 5/1959 Lafferty 313/356 x 2,899,591 8/1959 Stein 313/340 x 3.204.140 8/1965 Keams 313/239 Primary Examiner lames W. Lawrence Assistant Examiner-Saxfield Chatmon, Jr.

Attorney, Agent, or FirmW. H. MacAllister, Jr.; R. A. Cardenas [57] ABSTRACT 6 Claims, 4 Drawing Figures summrz PAIENIEllJun 4 m4 Flg 1 PRIOR ART PATENTEDJUN 4 I974 SHEET 2 0F 2 Fig. 4.

Room Temperature Number of Turns CATIIODE GUN DEVICE FIELD OF THE INVENTION This invention relates generally to traveling wave tubes (TWT) and more particularly to a TWT utilizing a foil insulated cathode that is maintained at thermionic emission temperature with substantially less power than previous insulated cathodes.

DESCRIPTION OFTHE PRIOR ART A traveling wave tube includes an electron gun assembly and a collector assembly coupled together by a slow-wave structure. The electron gun assembly emits a stream of electrons which travels through the slowwave structure and is collected by the collector assembly. The stream of electrons is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic energy. In order to achieve such interaction, the electromagnetic wave is propagated along the slow-wave structure, such as a conductive helix wound about the path of the electron stream. .The slow-wave structure provides a path of propagation for the electromagnetic wave such that the traveling wave effectively propagates at nearly the veelectric field may be applied for focusing the electron stream emitted from the cathode electrode. An accelerating anode may be used to accelerate the electrons before the stream enters the slow-wave structure. The above-mentioned electrical components are positioned in a supporting structure which is attached to one end of the slow-wave structure.

Thermionic emission from the cathode takes place when the cap is heated to approximately I,000C by the cathode heater. Along with the cathode heater and the cathode being heated to I,O00C the other components and the support structure are also heated to approximately the cathode temperature by both radiation and conduction from the cathode electrode. Since essentially the entire electron gun assembly is heated by the cathode heater to a very high temperature, sufficient power must be supplied to the cathode heater to maintain the cathode at the proper thermionic emission temperature. The requirement of additional power to maintain the entire cathode gun assembly at an ele vated temperature may place excessive demands on a limited power source such asin a space vehicle. Also, heating of components other than the cathode heater and the cathode induces problems of alignment of these other electrodes, i.e. metals expand causing position changes of these various electrodes making alignment difficult. High temperature operation may also tend to induce metal fatigue resulting in a shortened life for the cathode gun assembly.

Various prior art solutions for reducing the power requirements of a TWTs electron gun assembly have been tried with only limited success.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a simple, reliable and more efficient traveling wave tube.

It is another object of the present invention to provide a traveling wave tube electron gun assembly operating at a lower temperature.

It is a further object of the present invention to provide a cathode electrode requiring less power for thermionic emission.

It is a still further object of the present invention to provide an insulated cathode electrode that transfers a limited amount of heat to its surroundings.

It is still another object of the present invention to provide an insulated cathode electrode having a shield that is compact and stable under vibration.

In accordance with the foregoing objects, a cathode electrode, according to the invention, includes a cathode mounted to one end of a tubular cathode support member. A polished, embossed, thin foil made of a low thermal conductivity and low emissivity metal is coiled about the cathode electrode such that adjacent coils are separated from each other by the embossed surfaces. An effective plurality of individual heat shields is then formed wherein there is low thermal conductivity along the length of the foil and from coil to coil. The polished foil surface reflects heat back to a heatsource while a minimum of heat is radiated by the low emissivity foil.

The foregoing and other objects and features of the present invention will become readily apparent from the following description of preferred embodiments of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF TI-IE DRAWINGS FIG. 1 is a partial longitudinal sectional view illustrating a cathode gun as is known in the prior art;

FIG. 2 is a side view, partially broken away, of a cathode gun and heat shields according to an embodiment of the present invention;

FIG. 3 is a top view of the invention according to the embodiment of FIG. 2;

FIG. 4 is a graph of number ofcoil turns versus heat loss represented as temperature of an outside coil, for the parameters of: conduction, radiation, and a combination of conduction and radiation.

DETAILED DESCRIPTION OF THE DRAWINGS Referring more specifically to FIG. 1, a cathode gun device, according to the prior art, may be seen to include a cathode electrode 10, a heater electrode 20, anda heat reflective shield 30. The cathodeheater 20 heats the cathode electrode 10 to thermionic emission temperature, approximately 1,000C, andthe heat reflective shield 30, surrounding the exterior of the cathode electrode 10, reflects back some heat that is radiated from the circumference of the cathode electrode 10.

The cathode electrode 10 includes an electron emitting cathode cap 11, a cathode support member 14,

I and a cathode sleeve 16. The electron emissive cathode cap, hereinafter called cathode 11, is a cylindrically shaped disk with a concave surface at one end 12 from which electrons are emitted and a flat surface at the other end 13. The cathode 11 may be made of a material suitable for electron emission when heated to the proper thermionic temperature such as molybdenum or tantalum, for example. The flat end 13 of the cathode 11 is mounted to one end of a tubular cathode support member 14 which may be made of a metal such as molybdenum. The other end 15 of the cathode support member 14 is mounted to a tubular cathode sleeve 16 that is made of a material having lower thermal conductivity than the cathode support 14 such as tantalum. A heat reflective shield 30 is disposed about the circumference of the cathode 11, the cathode support member 14, and is attached to the cathode sleeve 16. The heat reflective shield 30 is tubular in shape with a substantial portion of its length having an inside diame ter larger than the cathode 11 or the cathode support member 14 such that the heat reflective shield 30 is spaced away from cathode 11 and the cathode support 14. One end of the heat reflective shield 30 has a sufficiently reduced diameter to be attached onto the cathode sleeve 16 by any convenient method such as spot welding, for example, along the circumference indicated as point 31.

Located within the cathode electrode 10 is the cathode heater 20 for heating the cathode 11 to thermionic emission temperature of approximately 1,000C. The cathode heater 20 is in close proximity to the cathode 11 for maximum heat transfer to the cathode 11. An insulating structure 21, such as a suitable high temperature potting compound, may be used to space the cathode heater 20 away from the cathode 11 and the cathode support 14. Terminals 22 and 23 of the cathode heater element 20 may extend beyond the cathode sleeve 16 for convenient electrical connection.

In operation of the device of FIG. 1, power is applied to the terminals 22 and 23 of the cathode heater 20 thereby heating the cathode 11 to thermionic emission temperature. The cathode support 14 and the cathode sleeve 16 are also heated by the cathode heater 20. Upon heating, these structures will radiate a substantial amount of heat, as much as 50 percent, away to their surrounds. With the heat reflective shield 30 in place, some of the outwardly radiated thermal energy is reflected back to the cathode electrode 10. However, the shield 30 will itself be heated as it conducts heat away from the area where the shield 30 is attached to the cathode sleeve 16. The shield 30 will then radiate this heat energy to its surroundings. lt is, therefore, apparent that even using such a heat shield there is still loss of heat and undue consumption of power due to unnecessary heating of the entire electron gun assembly by the cathode electrode 10. Also, such a heat reflective shield 30 is unstable during period of mechanical vibration. Since one end is unsupported, it is free to oscillate in response to mechanical vibration and cause mechanical and electrical malfunction of the electron gun assembly by shorting out some of the neighboring electrodes and structure.

Referring more specifically to FIG. 2, a cathode gun assembly according to the present invention may be seen to include a cathode electrode 10, a cathode heater electrode 20, a first heat shield 40, and a second heat shield 50. The cathode electrode 10 and the cathode heater electrode 20 are similar to the corresponding components described in FIG. 1 having the corresponding reference designations. Each of the first and second heat shields, 40 and 50 respectively, is an effective plurality of heat shields made of low thermal conductivity, low emissivity metal foil having a very small thickness and a polished surface. The first heat shield 40 is disposed about the outer circumference of the cathode electrode 10, while the second heat shield 50 is disposed about the inner circumference of the cathode electrode 10.

The first heat shield 40 is a thin embossed metal sheet or foil 41 attached to the outside diameter of the cathode electrode 10 and wound or coiled several times around the cathode electrode 10, thereby forming a plurality of layers of a spiral configuration about the cathode electrode 10. The leading edge of the foil 41 is attached to the cathode electrode 10 by welding or spot welding along the line 42. The foil 41 may be kept from unwinding by securing the outermost coil by welding or spot welding along the line 43 or by a sleeve 44, shown as a dotted line in H0. 2, being positioned over the heat shield 40. The foil 41 thickness in the illustrated arrangement is approximately 0.0002 inch. The embossing may be in any convenient pattern that projects or extends a substantially small portion of the foil surface above the rest of the surface. The projections may be dimples or trough-like depressions that are made with a punch and die. The embossing or projections, herein shown extending from the polished surface 47, may extend from either side of the foil. Projections may also be made in an additive manner by welding beads on one surface of the foil 41. The embossed pattern or the projections is such that a surface placed face-to-face on the projections is separated from the substantially flat surface of the embossed foil 41 by those projections. The apex of a typical projection is preferably relatively pointed or sharp so that it contacts a very small area on an adjacent contacting surface. The more coils that are placed about the cathode electrode the greater is the developed thermal insulation. Practically speaking, it has been determined that the best results are attained with about 40 turns and any additional turns improve the insulation characteristics a relatively small amount. However, within the scope of the invention, as few as 3 or 4 turns may be used to reduce the heat loss by the cathode electrode 10. The effectivity of reducing heat loss from the cathode electrode 10 is partially dependent upon the number of coils and is described below in FIG. 4.

An embossed pattern is provided on the foil 41 and the various coils about the cathode electrode 10 may be seen to be spaced away from each other. Each succeeding coil 41B is separated from the preceding coil 41A by a dimple 45 provided by the embossed pattern on the succeeding coil 418. The tip 46 of the dimples, such as 45, makes minimal contact with the preceding coil 41A and a very small amount of heat is transferred by conduction through such a small area from one coil to another.

The foil 41 has a polished inside surface 47 such that radiant thermal energy emitted by the cathode electrode 10 is reflected by the polished surface; and the foil may be made of a low emissivity material, in the neighborhood of e 0.], so that each coil radiates a small amount'of heat. The outer surface 48 need not be highly polished. The material should also have a low thermal conductivity constant such that heat transfer by conduction along the length of the foil from the innermost coil to the outermost is low. Materials having thermal conductivity constants ranging from 0.1 l l to 0.390 cal/cm /C/sec may be used with satisfactory results. Ideally also, the foil 41 should be very thin, approximately 0.0002 inch thick, for example, so that the area through which heat is conducted, i.e., along the length of the foil from the innermost to the outermost coil, is very small, thereby limiting heat conduction.

The second heat shield 50 is formed by attaching a foil 51 to the inside diameter of the cathode electrode l and winding the foil 51 several times within itself. The foil is welded to the cathode electrode 10 along the line 52 and the innermost coil is welded along the line 53 to prevent it from unwinding. In lieu of welding a sleeve 54 may be placed inside the heat shield 50 to prevent the foil 51 from unwinding. The second heat shield 50 may be of the same type of material as the first foil 41, i.e., thin, embossed, low thermal conductivity and low emissivity polished surface for providing thermal insulation similar to the first heat shield 10. It is pointed out that it is not necessary in accordance with the invention that the first and second heat shields, 40 and 50, respectively, be used in conjunction with each other, but they may be used individually.

A partial listing of suitable materials from which the foils 41 and 511 may be made is given below. These materials were chosen because they are typically employed in making cathode electrodes and they can withstand the high temperature requirements of normal cathodes. It is further pointed out the other materials may be used which fulfill the above-described requirements and the invention is not to be limited to those materials listed. Some of the materials that may be utilized for the foils 41 and 51 in practicing the invention include:

Material Thermal Emissivity e Conductivity cal/cm /cm/C/sec Rhenium 0.] ll 0.40 Columhium (H3 037 Tantalum 0.13 0.22 Molybdenum 0.32 0.l5 Tungsten 0.39 0.l75

The thermal conductivity constants given are at 0C, while the emissivity constants are at l,000C.

Referring now also to F IG. 3, a top view of the invention according to FIG. 2 includes the cathode electrode and a first heat shield 40 as shown in FIG. 2. The innermost coil is welded to the cathode electrode 10 along the line extending downwards (into the drawing) from point 42. The outermost coil is welded to the previous coil along the line extending downward (into the drawing) from the point 43. An alternate method of preventing the foil 411 from unwinding is by placing the sleeve 44, shown by a dashed outline, about or around the outer surface of the foil 41.

The operation of the invention according to FIGS. 2 and 3 will now be described with reference to two equations, relating to the transfer of thermal energy, and to FIG. 4. in order to aid in the understanding of the present invention, and the relation of a foils physical dimensions and constants to heat transfer by conductivity, ignoring radiation effect, the following equation is considered for a steady state condition:

Q=KA /LAT where Q heat in calories/sec; K thermal conductivity of the material in cal-cm/cm -sec-T;

A cross-sectional area of the foil in a plane orthogonal to the length of the foil strip in the direction of winding in cm (thickness x width along axis of cathode in the direction of the longitudinal center axis of the coil; L length of the foil along the length of the foil winding along which the heat flows in cm; and A T difference in temperature between the cathode temperature (approx. 1,000C) and room temperature (2pm;- 2 Assuming a predetermined temperature difference between the ends of a foil, it can be seen from the equa' tion that minimum heat flow between the ends is attached by minimizing K and the ratio A,/L. A minimum A /L ratio is obtained by minimizing the c sssectional area A, and maximizing the length L. AJIIS minimized by using a very thin foil, i.e., .0902 inch thick. The length is maximized by using a relatively long foil, i.e., long relative to the thickness, such as with a relatively large number of coils. Therefore, as a result of selecting these parameters, the ratio A /L may be made relatively small. Minimum heat flow along the length of the foil also requires using materials having a low thermal conductivity constant, K, as described above.

In order'to more fully explain the relation of emissivity to the invention, the second equation will now be used to describe thermal transfer by radiation only. For solving this equation, the foil 40, instead of being considered as a continuous coiled sheet, is considered as individual heat shields or coils coaxially disposed about each other and spaced away from each other, thereby ignoring the effects of thermal transfer by conduction. Since the coils have a polished inner surface, most of the heat that is radiated by the cathode electrode 10 will be reflected back to that cathode electrode 10. During steady state operation, the heat that is absorbed by the coils will be radiated outwardly according to the following equation:

where Q calories/sec; e emissivity for a particular metal; A, the emitting surface area of an individual coil in cm (width x circumference); a 1.35 X 10 cal/cm -sec-C; T absolute temperature of a coil in K; and T absolute temperature of an outward succeeding coil in K.

From this second equation, it is apparent that using a low emissivity material will limit the heat radiated outwardly from a coil to a minimal amount. Further, the highly polished inside surface 47 of the foil 41 reflects a substantial portion of the cathodes radiated heat back toward the cathode electrode 10.

In orderto more fully appreciate the significance of the above two equations and their relation to the present invention, reference is now made to FIG. 4. The coordinates of the graph represent steady state temperature of the outer surface of a coil in C versus the number of turns or coils that make up the total heat shield.

The solid curve 60 represents heat transfer by conduction along a continuously wrapped foil according to the first equation and is an approximation of the temperature of the outermost coil. The short dashed curve 70 represents heat transfer by radiation between coaxially disposed heat shields according to the second equation and is an approximation of the temperature of the outermost shield. The long dashed curve 80 represents an approximation of the temperature of the last coil due to the cumulative effects of both conduction and radiation according to both of the above equations. It is further pointed out that these curves are calculated approximations based on radiation or conduction only and are illustrative of the relationship among these various parameters and they are not illustrative of any absolute values.

At the origin, all curves begin at l,OOC, which is assumed to be the cathode ll temperature. The curve 60, the temperature due to heat conduction only, has a small slope relative to the other curves indicating that heat is being conducted along the foil to the outer coil and therefore there is a smaller temperature difference between coils. For the curve 70, the temperature curve initially has a steep slope indicating that energy incident to the coils inside surface is reflected back and little heat is radiated from the outside surface due to the low emissivity constant of the coil and there is no heat conduction between coils. The curve 80 is an approximation of the combined effects of both heat conduction and radiation on the temperature of the outside coil. It has been calculated and verified experimentally that as the number of coils about the cathode electrode approaches 40, the temperature of the outermost coil for all three curves asymtotically approaches room temperature. As the temperatures of successive coils are more nearly equal at about 40 coils, the heat losses due to conduction and radiation are about the same,

and therefore all three curves would approach the same temperature almost simultaneously.

It should be apparent from the foregoing that the present invention provides a simple and reliable shielded cathode electrode. Moreover, the device may reduce the input power required to maintain the cathode at thermionic emission temperature by as much as 50 percent. In fact, a cathode electrode according to FIGS. 2 and 3 was constructed and tested using 40 turns of a 0.0002 inch thick molybdenum foil to provide an outer shield. The tests revealed that an unshielded cathode electrode normally requiring 21.5 watts'of input power, with the shielded cathode requiring only 13.3 watts, and it is believed the temperature of the outside coil was slightly above ambient or room temperature.

Although the present invention has been shown and described with reference to particular embodiments, nevertheless, various changes and modifications obvious to one skilled in the art to which the invention pertains are deemed to lie within the purview of the invention.

What is claimed is:

l. A cathode gun device for use in a traveling-wave tube comprising:

a cathode for being heated to thermionic emission temperature;

a tubular cathode support member having one end coupled to said cathode;

a metal foil being coiled about itself several times and being disposed about said cathode support member forming an effective plurality of heat shields;

said foil having a substantially thin cross-sectional area for minimizing heat transfer by conduction perpendicular to said area;

said foil being relatively long in length for having a relatively long heat conduction path;

said foil having a relatively low thermal conductivity constant for minimizing thermal transfer by conduction along the length of said foil;

said foil having projections from a selected surface for substantially separating adjacent coils and minimizing thermal transfer by conduction therebetween;

said foil having a polished surface for effectively refleeting heat from said surface;

said foil having a relatively low emissivity for minimizing thermal transfer by radiation; and

means for mounting said foil to said cathode support member. I

2. The invention according to claim 1 wherein said means for mounting said foil comprises welding said foil to said cathode support member.

3. The invention according to claim 1 wherein said means for mounting said foil comprises a sleeve member positioned to secure said foil to said cathode support member.

4. The invention according to claim 1 wherein said metal foil is a metal selected from the group consisting of:

rhenium, columbium, tantalum, molybdenum and tungsten.

5. A cathode gun device for conserving power in a traveling-wave tube comprising:

a cathode for being heated to thermionic emission temperature;

a tubular cathode support member having said cathode mounted on one end;

a metal foil coiled about itselfa plurality of times and disposed about said cathode support member to form an effective plurality of heat shields;

said foil being relatively long in length for providing a relatively long heat conduction path;

said foil having a substantially thin cross-sectional area for minimizing heat transfer by conduction along the length of said foil;

said foil having a relatively low thermal conductivity constant for minimizing thermal transfer by conduction along the length of said foil;

said foil having projections from a surface for substantially separating adjacent coils and minimizing thermal transfer by conduction therebetween;

said foil having a polished surface for effectively refleeting heat from said surface; and

said foil having a relatively low emissivity for minimizing thermal transfer by radiation.

6. The invention according to claim 5 wherein said metal foil is a metal selected from the group consisting of:

rhenium, columbium, tantalum, molybdenum and tungsten. 

1. A cathode gun device for use in a traveling-wave tube comprising: a cathode for being heated to thermionic emission temperature; a tubular cathode support member having one end coupled to said cathode; a metal foil being coiled about itself several times and being disposed about said cathode support member forming an effective plurality of heat shields; said foil having a substantially thin cross-sectional area for minimizing heat transfer by conduction perpendicular to said area; said foil being relatively long in length for having a relatively long heat conduction path; said foil having a relatively low thermal conductivity constant for minimizing thermal transfer by conduction along the length of said foil; said foil having projections from a selected surface for substantially separating adjacent coils and minimizing thermal transfer by conduction therebetween; said foil having a polished surface for effectively reflecting heat from said surface; said foil having a relatively low emissivity for minimizing thermal transfer by radiation; and means for mounting said foil to said cathode support member.
 2. The invention according to claim 1 wherein said means for mounting said foil comprises welding said foil to said cathode support member.
 3. The invention according to claim 1 wherein said means for mounting said foil comprises a sleeve member positioned to secure said foil to said cathode support member.
 4. The invention according to claim 1 wherein said metal foil is a metal selected from the group consisting of: rhenium, columbium, tantalum, molybdenum and tungsten.
 5. A cathode gun device for conserving power in a traveling-wave tube comprising: a cathode for being heated to thermionic emission temperature; a tubular cathode support member having said cathode mounted on one end; a metal foil coiled about itself a plurality of times and disposed about said cathode support member to form an effective plurality of heat shields; said foil being relatively long in length for providing a relatively long heat conduction path; said foil having a substantially thin cross-sectional area for minimizing heat transfer by conduction along the length of said foil; said foil having a relatively low thermal conductivity constant for minimizing thermal transfer by conduction along the length of said foil; said foil having projections from a surface for substantially separating adjacent coils and minimizing thermal transfer by conduction therebetween; said foil having a polished surface for effectively reflecting heat from said surface; and said foil having a relatively low emissivity for minimizing thermal transfer by radiation.
 6. The invention according to claim 5 wherein said metal foil is a metal selected from the group consisting of: rhenium, columbium, tantalum, molybdenum and tungsten. 